1. Field of the Invention
The present invention relates to a thermal recording apparatus and a process therefor, and more particularly to a thermal recording apparatus and a process therefor adapted for use in the recording unit of a facsimile apparatus or the like.
2. Related Background Art
In the conventional thermal recording process, the time required for, moving a recording sheet by a step is selected to be approximately equal to the time required for recording a line, in order to improve the efficiency in time. In such process there is employed so-called "movement printing" in which the printing operation is conducted simultaneously with the triggering of the stepping motor for a one-step movement thereof. Such movement printing is employed even in the facsimile apparatus of the G1 or the G2 mode with a relatively long printing cycle.
Also in such Gl or G2 mode, there is conventionally employed a method of repeating the step of black color generation plural times for the same line, in order to increase the density of black color. Herein, black color generation means that a black image or dot appears on a heat sensitive recording sheet by heating such a sheet. This method allows one to increase the color density without loss in time, utilizing the relatively long interval between the lines in said Gl or G2 mode. However, this method results in an enlargement in the printed area in the case of the movement printing, since the first printing operation is conducted during the movement of the recording sheet while the second printing operation is conducted after the one-step movement of said sheet.
Such enlargement of the image area tends to obscure fine image patterns.
FIGS. 4A and 4B show the above-explained printing operation and the movement of the stepping motor.
At first reference is made to FIG. 4A for explaining the timing of movement printing. A CPU supplies the stepping motor with a trigger pulse for starting a one-step movement, and a printing operation is conducted. It is assumed that the duration of the printing pulse is 5 msec. In response to said trigger pulse, the stepping motor effects the movement of a step, which is normally equal to 1/3.85 mm. This movement also takes about 5 msec. but involves an oversheet, and requires additional time for the complete movement.
The printing operation is conducted after the emission of the trigger pulse for the stepping motor, in order to maintain a constant relationship between the moving distance provided by the motor and the timing of printing operation regardless of the interval of printing operations.
FIG. 4B is a timing chart for a case in which the printing operation is repeated twice in order to increase the density of black color generation in the G1 or G2 mode.
As shown in FIG. 4B, the first printing operation is conducted during the movement by the motor, while the second printing operation is conducted after said movement is almost completed.
FIG. 5 shows a case of printing a line in four blocks with a thermal head the length of the thermal element of which is equal to the length of one step (1/3.85 mm).
The first printing operation, conducted during movement, generates black color over a length of 1/3.85 mm. The second printing operation, conducted in a stopped state, generates black color over a length of 1/3.85 mm around the center of the heating element, after the head movement, in every block. As will be understood from FIG. 5, the enlargement of printed area is larger in the earlier blocks.
Also in the thermal printer of a facsimile apparatus designed for Gl, G2 and G3 modes, said "movement printing" is employed in all the moves, by conducting the printing operation after the movement of the recording sheet is started.
However, though such control timing works satisfactorily in the G3 mode with a fast transmission speed and with a short printing cycle, it tends to result in an undesirable phenomenon of sticking in the Gl or G2 mode with longer printing cycles. The sticking phenomenon means the adhesion of the thermal recording sheet and the heating element of the thermal recording head, and tends to occur when the printing operation is conducted by activating a heating element maintained in contact with the thermal recording sheet and terminating the activation of the heating element without moving the recording sheet. Also said sticking becomes more marked as the number of heating dots in a line increases, or the temperature difference of the thermal head between the heated state and the cooled state increases. In the aforementioned timing control in which the printing operation is conducted after the movement of the recording sheet is started, therefore, the sticking tends to occur in the G1 or G2 mode in which the recording cycle is longer so that the heat dissipation of the thermal head is larger. The sticking between the thermal recording sheet and the thermal head leads to various troubles such as an increased load on the motor for moving the recording sheet, an increased noise level in the sheet movement, white streak formation on the printed image, or glossy surface formation of the thermal recording sheet.